Process and electrolyte for coloring aluminum



United" States Patent 3,384,562 PROCESS AND ELECTROLYTE FOR COLORING ALUMINUM Christian E. Michelson, Hamden, and David C. Montgomery, Clinton, Conn., assignors to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Feb. 15, 1965, Ser. No. 432,889

7 Claims. (Cl. 204-58) ABSTRACT OF THE DISCLOSURE The instant disclosure teaches a process and electrolyte for coloring aluminum electrolytically which achieves a wide variety of light stable surface colors having excellent physical characteristics, with the process characterized by anodically oxidizing aluminum in an aqueous solution consisting essentially of from 0.1 to 1% sulfuric acid and from 3% to saturation of sulfamic acid.

The present invention relates to anodizing aluminum. More particularly, the present invention resides in the production of integral, colored, anodic coatings on an aluminum article by making the aluminum article anode in an electrolytic cell utilizing a particular aqueous acid electrolyte.

Decorative and protective oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of aqueous solutions of sulfuric acid. Such electrolytes are usually employed at or near room temperature and must be cooled to maintain such temperature. The coatings so produced are usually relatively clear or colorless, although on some alloys they may be tinted or colored by some constituents, and the abrasion resistance is in general not very great.

Processes have been developed for producing anodic oxide coatings which are more abrasion resistant by anodizing in aqueous sulfuric acid electrolytes at extremely low temperatures, i.e., from about 0 to 30 F. These processes are subject to the inherent disadvantage of requiring expensive refrigeration equipment to maintain the temperature below 30 F. The coatings so produced, however, in addition to being abrasion resistant are attractively colored for numerous uses of aluminum, i.e., are colored in dark desirable colors such as dark brown, etc.

It is, therefore, highly desirable to develop a practical process for anodically obtaining relatively dark surface colors and good corrosion resistance.

Accordingly, it is an object of the present invention to provide a practical process for producing an integral, colored, anodic coating on an aluminum article and an electrolytic cell for achieving said process.

It is a further object of the present invention to provide a process as aforesaid for anodizing aluminum which yields oxide coatings having relatively dark surface colors, in particular, colors which are light stable.

In addition, it is an object of the present invention to provide a process as aforesaid which yields an integral, colored, aluminum article having good physical properties, such as excellent corrosion resistance.

It is a still further object of the present invention to provide a process as aforesaid which enables the attainment of a wide variety of the highly desirable integral dark surface colors.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the process of the present invention it has now been found that the foregoing objects and advantages may be readily obtained. The present invention provides a process for coloring aluminum electrolytically, said process comprising anodically oxidizing ice said aluminum at a current density of from 10 to 100 amps per square foot and a voltage of from 10 to volts in an aqueous solution consisting essentially of from 0.1 to 1% sulfuric acid and from 3% to saturation of sulfamic acid, preferably from 5 to 15% sulfamic acid, said bath being maintained at a temperature of from 10 to 90 C.

It has been found in accordance with the present invention that the foregoing process surprisingly achieves all of the objects of the present invention and readily achieves a wide variety of light stable surface colors having excellent physical characteristics and varying from light silver gray to jet black or a variety of browns and dark olive shades.

It is particularly surprising that the present process attains a wide range of highly desirable dark surface colors. One particularly unexpected feature of the present invention is the production of integral dark colors over a relatively wide range of sulfamic acid concentrations but over a very narrow range of sulfuric acid concentrations. As will be shown in the examples which form a part of the present specification, the varying of the sulfuric acid concentration has a pronounced effect.

An additional and significant feature of the present invention is that all of the foregoing advantages are obtained at a low cost which renders the process particularly attractive commercially.

It has been found that the process variables of the present invention are important in attaining the desired eifect.

The sulfamic acid concentration may be varied over a wide range of from 3% to saturation and preferably should be maintained in the range of 5% to 15 The sulfuric acid concentration is particularly critical and should be maintained in the narrow range of from 0.1 to 1% by weight and preferably from 0.25 to 0.75% by weight.

The aqueous bath is maintained at a temperature of from 10 to 90 C. and preferably from 15 to 30 C.

Naturally, as is known in the art, additional substituents may be added to the bath to achieve particular results, such as, for example, metal sulfates.

The aluminum article is anodically oxidized at a current density of from 10' to amps per square foot and preferably at a current density of 15 to 30 amps per square foot and at a voltage of from 10 to 90 volts, and preferably at a voltage of from 25 to 70 volts.

The time of oxidation is not especially critical, with longer oxidation times yielding thicker coatings and darker colors. In general, however, an anodizing time of at least one minute is employed and preferably from 1 to minutes.

The present invention is applicable to anodizing any aluminum article, i.e., high purity aluminum, aluminum in various commercial grades, and aluminum base alloys.

If desired, after the anodic treatment the aluminum article may be sealed by various conventional treatments, e.g., immersion in hot water maintained at or near the boiling point.

In the electrolytic cell, the cathode which may be used is not especially critical. conventionally, lead or preferably stainless steel cathodes may be used.

The present invention and the improvements resulting therefrom will be more readily apparent from a consideration of the following illustrative examples wherein all materials are present in percents by weight.

Example I The following example illustrates the wide variety of colors which may be obtained on a number of alloys in accordance with the process of the present invention.

In the following example various aluminum alloys were anodically oxidized for varying periods of time at specified current densities and voltages. The electrolyte used was an aqueous solution consisting essentially of about 0.25% sulfuric acid and about 10% sulfamic acid. The bath was maintained at a temperature of 25 C. The cathode used was lead. The voltage was raised to maintain the current density until the maximum voltage was reached. The voltage was then left constant until the anodizing time was completed. Air agitation of the electrolyte was used.

The results are shown in the Table I below:

The following example illustrates the surprising effect of varying the sulfuric acid concentration.

In the following example aluminum alloy 6061-"16 was anodically oxidized in a manner after Example I at a current density of 24 amps per square foot. This required a voltage varying between about 40 volts at the start of the process up to a maximum of about 50 volts by the end of about 10 minutes. After the 10 minute period the voltage was maintained at about 50 volts and the current allowed to drop.

In this example aqueous electrolytes were used containing varying concentrations of sulfuric acid and sulfamic acid. The resulting colors obtained are shown in photovolt readings wherein generally darker colors are indicated by low photovolt readings, i.e., the lower photovolt reading indicates lower reflectivity, hence a darker color.

These results show graphically that the darker colors are obtained in accordance with the process of the present invention. The results are shown in Table II below:

Example III The following example again illustrates the criticality of the process conditions of the present invention.

In the following example various aluminum alloys were anodically oxidized in a manner after Example II using a current density of 12 amps per square foot and utilizing (1) an aqueous electrolyte containing sulfamic acid and 0.5% sulfuric acid, and (2) an aqueous electrolyte containing only 165 grams per liter sulfuric acid.

The results are given in Table III below and indicate the alloy used, the voltage and treatment times and the photovolt readings for each respective electrolyte. These examples clearly show that the desirable dark colors are obtained in accordance with the process of the present invention.

TABLE III Max- Aluminum Alloy, Aluminum Elec- Time, lmum Photo- Association Designation tro- Min- Voltvolt.

utcs age, Reading Volts Example IV Aluminum alloy 6061, two samples, was anodically colored as in Example I using the conditions shown in Table I for alloy 6061. The colors obtained were brown and black, respectively, as shown in Table I.

These samples were then subjected to 2500 hours of ultraviolet light exposure. After the 2500 hours exposure, both samples retained their brown and black colors.

Example V The following example-s illustrate the abrasion resistance of the samples anodized in accordance with the process of the present invention.

Samples anodized in a manner after Example I were tested for abrasion resistance by abrading the surface of the sample with a fine jet of sand. The time necessary to penetrate the anodic coating in ten (10) spots on each specimen was measured. The total time to penetrate the ten (10) spots was then divided by the coating thickness in mils, giving a figure of merit the units of which are seconds per miLThe higher the seconds per mil, the greater the abrasion resistance.

The data is shown in the following table.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. A process for coloring aluminum electrolytically which comprises anodically oxidizing said aluminum at a current density of from to 100 amps per square foot and a voltage of from 10 to 90 volts in an aqueous solution consisting essentially of from 0.1 to 1% sulfuric acid and from 3% to saturation of sulfamic acid, said bath being maintained at a temperature of from 10 to 90 C.

2. A process according to claim 1 wherein the sulfamic acid is present in a concentration of from 5 to 3. A process according to claim 1 wherein the sulfuric acid is present in a concentration of from 0.25 to 0.75%.

4. A process according to claim l'wherein said bath is maintained at a temperature of from 15 to C.

5. A process according to claim 1 wherein the current density is from 15 to 30 amps per square foot and the voltage is from 25 to volts.

References Cited UNITED STATES PATENTS 2,855,352 10/1958 Ernst 204-58 JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner. 

